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Abstract:

The alloy fine particles of the present invention are fine particles of a
solid solution alloy, in which a plurality of metal elements are mixed at
the atomic level. The production method of the present invention is a
method for producing alloy fine particles composed of a plurality of
metal elements. This production method includes the steps of: (i)
preparing a solution containing ions of the plurality of metal elements
and a liquid containing a reducing agent; and (ii) mixing the solution
with the liquid that has been heated.

Claims:

1-18. (canceled)

19. Fine particles of a solid solution alloy, in which a plurality of
metal elements are mixed at the atomic level wherein the plurality of
metal elements are a plurality of metal elements that do not form a solid
solution even in the liquid phase.

20. The alloy fine particles according to claim 19, wherein the plurality
of metal elements are two kinds of metal elements.

21. The alloy fine particles according to claim 20, wherein elemental
mapping using a scanning transmission electron microscope with a
resolution of 0.105 nm demonstrates that there is no phase separation in
the alloy fine particles.

22. The alloy fine particles according to claim 20, wherein X-ray
diffraction demonstrates that there is no phase separation in the alloy
fine particles.

23. The alloy fine particles according to claim 21, wherein the plurality
of metal elements are silver and rhodium.

24. The alloy fine particles according to claim 21, wherein the plurality
of metal elements are gold and rhodium.

25. The alloy fine particles according to claim 19, having an average
particle size of 20 nm or less.

26. A method for producing alloy fine particles composed of a plurality
of metal elements, the method comprising the steps of: (i) preparing a
solution containing ions of the plurality of metal elements and a liquid
containing a reducing agent; and (ii) mixing the solution with the liquid
that has been heated, wherein the plurality of metal elements are a
plurality of metal elements that do not form a solid solution even in the
liquid phase.

27. The production method according to claim 26, wherein the reducing
agent is at least one type of alcohol selected from the group consisting
of ethylene glycol, diethylene glycol, and triethylene glycol.

28. The production method according to claim 26, wherein the reducing
agent is ethylene glycol, and in the step (ii), the solution is mixed
with the liquid that has been heated to a temperature not lower than a
temperature at which each of the ions of the plurality of metal elements
is reduced.

29. The production method according to claim 28, wherein in the step
(ii), the solution is mixed with the liquid that has been heated to a
temperature higher by 20.degree. C. or more than a temperature at which
each of the ions of the plurality of metal elements is reduced.

30. The production method according to claim 26, wherein the reducing
agent is ethylene glycol, the plurality of metal elements are silver and
rhodium, and in the step (ii), the solution is mixed with the liquid that
has been heated to 145.degree. C. or higher.

31. The production method according to claim 26, wherein the plurality of
metal elements are gold and rhodium.

32. The production method according to claim 26, wherein at least one
selected from the liquid and the solution contains a protective agent for
preventing agglomeration of the alloy fine particles.

33. The production method according to claim 26, wherein in the step
(ii), the solution is mixed with the liquid by dropping the solution into
the liquid.

34. The production method according to claim 26, wherein in the step
(ii), the solution is mixed with the liquid by spraying the solution onto
the liquid.

35. Alloy fine particles produced by the production method according to
claim 26, wherein the alloy fine particles are fine particles of a solid
solution alloy, in which a plurality of metal elements are mixed at the
atomic level.

Description:

TECHNICAL FIELD

[0001] The present invention relates to solid solution alloy fine
particles and a method for producing the same.

BACKGROUND ART

[0002] Alloys exhibit different properties from those of individual
constituent metal elements. Therefore, newly developed alloys are
expected to have properties (for example, catalytic properties) that
conventional metals do not have. On the other hand, metal fine particles
are expected to have a variety of applications for reasons such as their
large specific areas and possibly different properties and structures
from those of bulk metals. For these reasons, various alloy fine
particles have been studied. For example, a method for producing alloy
particles containing silver and rhodium is disclosed (see Non Patent
Literature 1).

[0005] As shown in a phase diagram of FIG. 18, however, silver and rhodium
in bulk form do not form a solid solution at the atomic level. Even if a
mixture of silver and rhodium is heated and melted, silver and rhodium
remain separated. Therefore, even if a melt containing silver and rhodium
is cooled rapidly, it is difficult to produce an alloy in which silver
and rhodium form a solid solution. On the other hand, in the method of
Non Patent Literature 1, silver ions and rhodium ions are reduced in a
solution to produce fine particles. In this method of Non Patent
Literature 1, however, it is difficult to produce fine particles in which
silver and rhodium form a solid solution at the atomic level.
Nevertheless, if silver and rhodium do not form a solid solution at the
atomic level, the resulting alloy is unlikely to exhibit its unique
properties. FIG. 19 shows a phase diagram of gold and rhodium. As is
clear from the phase diagram of FIG. 19, it is difficult to produce a
solid solution alloy of gold and rhodium.

[0006] Under these circumstances, it is one of the objects of the present
invention to provide alloy fine particles in which a plurality of metal
elements are mixed at the atomic level and a method for producing the
same.

Solution to Problem

[0007] In order to achieve the above object, the alloy fine particles of
the present invention are fine particles of a solid solution alloy, in
which a plurality of metal elements are mixed at the atomic level. The
phrase "mixed at the atomic level" means that, in one aspect, individual
elements are randomly dispersed in an elemental map obtained using a STEM
with a spatial resolution of 0.105 nm, and in another aspect, a single
peak pattern is observed by XRD.

[0008] The production method of the present invention is a method for
producing alloy fine particles composed of a plurality of metal elements.
This production method includes the steps of: (i) preparing a solution
containing ions of the plurality of metal elements and a liquid
containing a reducing agent; and (ii) mixing the solution with the liquid
that has been heated.

Advantageous Effects of Invention

[0009] According to the present invention, solid solution alloy fine
particles in which a plurality of metal elements are mixed at the atomic
level are obtained.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 shows an example of a step of the production method of the
present invention.

[0011] FIG. 2 shows another example of the step of the production method
of the present invention.

[0029] Hereinbelow, embodiments of the present invention will be described
by way of examples. The present invention is not limited to the following
embodiments and examples. In the following description, specific
numerical values or specific materials may be given by way of examples,
but other numerical values or other materials may be used as long as the
effect of the present invention can be obtained.

[0030] [Production Method of Alloy Fine Particles]

[0031] The method of the present invention is a method for producing alloy
fine particles composed of a plurality of metal elements. According to
this production method, solid solution alloy fine particles in which a
plurality of metal elements are mixed at the atomic level are obtained.
The alloy fine particles obtained by this production method constitute
one aspect of the alloy fine particles of the present invention.

[0032] The method of the present invention includes the following steps
(i) and (ii). Hereinafter, the plurality of metal elements that
constitute the alloy fine particles may sometimes be referred to as "a
plurality of metal elements (E)".

[0033] In the step (i), a solution containing ions of the plurality of
metal elements (E) and a liquid containing a reducing agent are prepared.
Hereinafter, the solution containing the plurality of metal elements (E)
may sometimes be referred to as a "metal ion solution" or a "solution
11". The liquid containing a reducing agent may sometimes be referred to
as a "liquid 12".

[0034] The plurality of metal elements (E) may be two kinds of metal
elements. In that case, binary alloy fine particles are obtained. When
the plurality of metal elements (E) include rhodium, rhodium alloy fine
particles are obtained.

[0035] An example of the plurality of metal elements (E) is a combination
of silver (Ag) and rhodium (Rh). Another example of the plurality of
metal elements (E) is a combination of gold (Au) and rhodium (Rh).

[0036] The metal ion solution can be prepared by dissolving at least one
type of compound containing the plurality of metal elements (E) in a
solvent. One compound may contain all the metal elements included in the
plurality of metal elements (E). One compound also may contain only one
metal element included in the plurality of metal elements (E).

[0037] When the plurality of metal elements (E) are silver and rhodium,
the metal ion solution can be prepared by dissolving a silver compound
and a rhodium compound in a solvent. Examples of the silver compound
include silver (I) acetate (AgCH3COO) and silver nitrate
(AgNO3). Examples of the rhodium compound include rhodium (III)
acetate(Rh(CH3COO)3) and rhodium (II) acetate
(Rh(CH3COO)2). As the solvent, a solvent capable of dissolving
silver ions and rhodium ions is used. An example of the solvent is water.

[0038] When the plurality of metal elements (E) are gold and rhodium, the
metal ion solution can be prepared by dissolving a gold compound and a
rhodium compound in a solvent. Examples of the gold compound include
chloroauric acid (HAuCl4). Examples of the rhodium compound include
the above-mentioned rhodium compounds and rhodium (III) chloride
(RhCl3). An example of the solvent is water.

[0039] The concentration of ions of one of the plurality of metal elements
(for example, silver ions or gold ions) in the metal ion solution may be
in the range of 0.1 mmol/L to 1 mol/L (for example, in the range of 0.1
mmol/L to 5 mmol/L). The concentration of rhodium ions in the metal ion
solution may be in the range of 0.1 mmol/L to 1 mol/L (for example, in
the range of 0.1 mmol/L to 5 mmol/L or in the range of 0.1 mmol/L to 1
mmol/L).

[0040] The alloy composition can be varied by varying the ratio between
the concentration of silver ions CAg (mol/L) in the metal ion
solution and the concentration of rhodium ions CRh (mol/L) in the
metal ion solution. The value of CRh/[CRh+CAg] may be 0.1
or more, 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more, 0.6 or more,
0.7 or more, 0.8 or more, or 0.9 or more. The value may also be 0.9 or
less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less,
0.3 or less, 0.2 or less, or 0.1 or less. The use of a metal ion solution
with CRh/[CRh+CAg]=X makes it possible to produce alloy
fine particles having a rhodium content of almost 100× atomic %.
For example, with the use of a metal ion solution with a
CRh/[CRh+CAg] value of 0.5 or more, alloy fine particles
having a rhodium content of 50 atomic % or more can be produced. When the
plurality of metal elements (E) are two kinds of elements, the
relationship between the concentrations of ions of the individual
elements in the metal ion solution and the resulting alloy composition is
the same as the above-mentioned relationship between the concentrations
of silver ions and rhodium ions and the resulting alloy composition.

[0041] Next, in the step (ii), the metal ion solution (solution 11) is
mixed with the heated liquid (liquid 12) containing a reducing agent. In
the step (ii), not only the liquid 12 but also the solution 11 may be
heated.

[0042] In the step (ii), the solution 11 may be mixed with the liquid 12
by dropping the solution 11 into the heated liquid 12. In the step (ii),
the solution 11 may be mixed with the liquid 12 by spraying the solution
11 onto the heated liquid 12. For example, in the step (ii), as shown in
FIG. 1, the solution 11 and the heated liquid 12 may be mixed by dropping
the former into the latter. In the step (ii), as shown in FIG. 2, the
solution 11 and the heated liquid 12 may be mixed by spraying the former
onto the latter. When the concentration of metal ions in the metal ion
solution is high, it may be preferable in some cases to mix the solution
11 and the liquid 12 by spraying the solution 11.

[0043] In the step (ii), the solution 11 may be mixed with the liquid 12
that has been heated to a temperature not lower than a temperature at
which each of the ions of the plurality of metal elements (E) is reduced.
Furthermore, in the step (ii), the solution 11 may be mixed with the
liquid 12 that has been heated to a temperature higher by 20° C.
or more than a temperature at which each of the ions of the plurality of
metal elements (E) is reduced. In these two cases, the reducing agent may
be ethylene glycol.

[0044] Spraying of the solution 11 and/or the liquid 12 can be performed,
for example, using a spray gun or an ink jet head. The particle size of
the alloy fine particles to be formed may possibly be controlled by
varying the size of sprayed droplets.

[0045] The reducing agent contained in the liquid 12 may be alcohol. The
liquid 12 as a solvent may consist of an alcohol acting as a reducing
agent (for example, ethylene glycol). The liquid 12 may contain an
alcohol not acting as a reducing agent in addition to an alcohol acting
as a reducing agent. When the liquid 12 is heated, the action of the
alcohol as a reducing agent is increased. The temperature to which the
liquid 12 is heated in the step (ii) depends on the type of alcohol as a
reducing agent contained in the liquid 12. For example, when ethylene
glycol is used, it is assumed that silver ions are reduced at 100°
C. or lower and rhodium ions are reduced at around 140° C.
Therefore, when the plurality of metal elements (E) are silver and
rhodium and ethylene glycol is used as a reducing agent, the liquid 12
must be heated to 140° C. or higher.

[0046] There is no limitation on the type of alcohol as a reducing agent
contained in the liquid 12 as long as the effect of the present invention
can be obtained. The alcohol used as a reducing agent may be a monovalent
alcohol, or a polyvalent alcohol such as a divalent alcohol. Preferred
examples of the alcohol used as a reducing agent are at least one type of
alcohol selected from the group consisting of ethylene glycol, diethylene
glycol, and triethylene glycol. Hereinafter, at least one type of alcohol
selected from the group consisting of ethylene glycol, diethylene glycol,
and triethylene glycol may sometimes be referred to as "ethylene
glycols". The boiling point of ethylene glycols is 190° C. or
higher. Therefore, the use of such an alcohol as a solvent makes it
possible to produce alloy fine particles at a high temperature.

[0047] Instead of the alcohol (i.e. an alcohol acting as a reducing agent)
contained in the liquid 12, a substance capable of reducing metal ions
(such as silver ions, rhodium ions, and gold ions) and acting as a
solvent may be used.

[0048] At least one selected from the solution 11 and the liquid 12 may
contain a protective agent for preventing the agglomeration of the alloy
fine particles. The use of a protective agent makes it easier to obtain
alloy fine particles of small size. Specifically, both or either one of
the solution 11 and the liquid 12 may contain a protective agent.
Examples of the protective agent include polymers and surfactants. For
example, the protective agent is poly(N-vinyl-2-pyrrolidone) (hereinafter
may be referred to as "polyvinylpyrrolidone" or "PVP"). The concentration
of the protective agent in the solution is selected according to the type
of the protective agent. When the protective agent is
polyvinylpyrrolidone, it may be added so that the concentration of its
constituent units (monomer units) is in the range of 0.1 mmol/L to 2
mol/L (for example, in the range of 1 mmol/L to 10 mmol/L).

[0049] When neither the solution 11 nor the liquid 12 contains a
protective agent, the alloy fine particles are likely to agglomerate to
form particles of larger size.

[0050] A typical example of the liquid 12 is a solution (an alcohol
solution of a protective agent) obtained by dissolving the protective
agent in an alcohol (for example, ethylene glycols). For example, an
ethylene glycol solution in which polyvinylpyrrolidone is dissolved can
be used as the liquid 12. Hereinafter, the liquid 12 in which the
protective agent is dissolved may sometimes be referred to as a "reducing
agent solution".

[0051] In one example, the reducing agent is ethylene glycol, the
plurality of metal elements (E) are silver and rhodium, and in the step
(ii), the solution 11 is mixed with the liquid 12 that has been heated to
145° C. or higher. In another example, the reducing agent is
ethylene glycol, the plurality of metal elements (E) are gold and
rhodium, and in the step (ii), the solution 11 is mixed with the liquid
12 that has been heated to 145° C. or higher. In these examples,
the liquid 12 may be an ethylene glycol solution in which
polyvinylpyrrolidone is dissolved. The solution 11 may be an aqueous
solution containing silver ions and rhodium ions or an aqueous solution
containing gold ions and rhodium ions.

[0052] In the production method of the present invention, the liquid 12
may be one essentially or substantially free from a reducing agent (for
example, sodium borohydride (NaBH4) or hydrazine) other than
alcohol. However, sodium borohydride or the like may be used as a
reducing agent as long as the effect of the present invention can be
obtained.

[0053] When the alcohol contained in the liquid 12 is ethylene glycol, the
liquid 12 may be heated to a temperature of 145° C. or higher. It
may also be heated to a temperature of 150° C. or higher or
160° C. or higher. In the step (ii), the liquid 12 may be heated
to a lower temperature as long as the effect of the present invention can
be obtained. In the step (ii), the liquid 12 may be heated to a
temperature of 200° C. or lower, for example, 50° C. or
lower.

[0054] In the step (ii), the solution 11 and the liquid 12 are mixed in
such a way that the temperature of the liquid 12 does not drop
excessively. For example, when the alcohol is ethylene glycol, the
solution 11 and the liquid 12 are mixed in such a way that the
temperature of the liquid 12 is maintained at 145° C. or higher,
150° C. or higher, or 160° C. or higher. A way of
preventing an excessive drop in the temperature of the liquid 12 is, for
example, to add the solution 11 little by little. Examples of methods of
adding the solution 11 little by little include a method of dropping the
solution 11 and a method of spraying the solution 11. The solution 11 may
also be added after it is heated to a certain temperature.

[0055] In one example, the weight of the solution 11 to be added per
second to the liquid 12 may be not more than one three-hundredth (for
example, not more than one three-thousandth) of the weight of the liquid
12.

[0056] According to the production method of the present invention, solid
solution alloy fine particles in which the plurality of metal elements
(E) are mixed at the atomic level are obtained. For example, fine
particles of a silver-rhodium solid solution alloy, in which silver and
rhodium are mixed at the atomic level, are obtained. Silver and rhodium
in bulk form do not form a solid solution at the atomic level. However,
fine particles having a particle size of several tens of nanometers or
less have different structures and properties from those of bulk metals,
and it is believed that silver and rhodium therein can form a solid
solution at the atomic level. Furthermore, according to the present
invention, fine particles of a gold-rhodium solid solution alloy, in
which gold and rhodium are mixed at the atomic level, are obtained.

[0057] Even if the plurality of metal elements (E) are a plurality of
metal elements that do not form a solid solution even in the liquid phase
in a phase diagram, the production method of the present invention makes
it possible to obtain alloy fine particles in which the plurality of
metal elements (E) form a solid solution at the atomic level. In this
case, the metal ion solution may be a solution containing a plurality of
metal elements whose concentrations correspond to the composition ratio
of those metals in bulk form that do not form a solid solution. This
production method makes it possible to obtain alloy fine particles in
which a plurality of metal elements form a solid solution at the atomic
level although these metal elements have a composition ratio that does
not allow them to form a solid solution in the liquid phase if they are
in bulk form (i.e., these metal elements include a plurality of metal
elements that do not form a solid solution in the liquid phase over the
entire range of composition ratios when they are in bulk form). The
production method of the present invention can be used for producing
various alloy fine particles.

[0058] [Alloy Fine Particles]

[0059] The alloy fine particles of the present invention are alloy fine
particles in which the plurality of metal elements (E) form a solid
solution. More specifically, the alloy fine particles of the present
invention are solid solution alloy fine particles in which the plurality
of metal elements (E) are mixed at the atomic level. The fact that they
are solid solution alloy fine particles in which the plurality of metal
elements (E) are mixed at the atomic level can be confirmed by
measurements or the like performed in the following examples. Examples of
the alloy fine particles of the present invention include rhodium alloy
fine particles containing rhodium. Examples of the alloy fine particles
of the present invention include silver-rhodium alloy fine particles and
gold-rhodium alloy fine particles.

[0060] The alloy fine particles of the present invention can be produced
by the production method of the present invention. Since the details of
the production method of the present invention that have been described
can be applied to the alloy fine particles of the present invention,
overlapping descriptions may be omitted. Furthermore, the details of the
alloy fine particles of the present invention that have been described
can be applied to the production method of the present invention.

[0061] In one aspect, the alloy fine particles of the present invention
are such that elemental mapping using a scanning transmission electron
microscope with a resolution of 0.105 nm demonstrates that there is no
phase separation in the alloy fine particles.

[0062] The alloy fine particles of the present invention (for example,
binary alloy fine particles) may be such that all of the plurality of
metal elements (E) are contained in any cube with a side length of 1 nm
that is arbitrarily selected from the alloy fine particle.

[0063] In one aspect, the alloy fine particles of the present invention
are such that X-ray diffraction demonstrates that there is no phase
separation in the alloy fine particles.

[0065] There is no limitation on the particle size of the alloy fine
particles of the present invention as long as the plurality of metal
elements (E) form a solid solution at the atomic level. The alloy fine
particles of the present invention (for example, silver-rhodium alloy
fine particles and gold-rhodium alloy fine particles) may have an average
particle size of 30 nm or less, 20 nm or less, or 10 nm or less. The
average particle size may be 3 nm or more. The average particle size can
be calculated in a manner described in the examples.

[0066] The alloy fine particles of the present invention may be composed
of the plurality of metal elements (E) that do not form a solid solution
even in the liquid phase.

[0067] The alloy fine particles of the present invention may contain trace
amounts of impurities as long as they do not essentially change the
properties of the particles.

EXAMPLES

[0068] Hereinafter, the present invention will be described in more detail
by way of examples. In the examples and comparative examples below, an
electron microscope (JEM 2010EFE manufactured by JEOL Ltd.) and a
scanning transmission electron microscope (HD-2700 manufactured by
Hitachi High-Technologies Corporation) were used for EDX measurements. An
X-ray diffractometer (D8 ADVANCE manufactured by Bruker AXS) and SPring-8
BL02B2 were used for XRD measurements. As a scanning transmission
electron microscope, HD-2700 with a resolution of 0.105 nm, manufactured
by Hitachi High-Technologies Corporation, was used. Elemental mapping was
conducted with EDX. In the following examples, elemental mapping data
were obtained using a scanning transmission electron microscope
(HD-2700). In the elemental mapping performed in the following examples,
an electron beam was scanned in two dimensions using the STEM to generate
a scan image while the EDX incorporated in the STEM detected the
elements, which were plotted in two dimensions with reference to the
operation of the STEM to conduct the elemental mapping.

Example 1

[0069] In Example 1, silver-rhodium alloy fine particles were produced by
dropping the solution 11.

[0071] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 160° C., the
metal ion solution was dropped with a syringe into the reducing agent
solution. At this time, the reducing agent solution was dropped in such a
way that the temperature of the reducing agent solution was maintained at
160° C. or higher. Next, the reducing agent solution into which
the metal ion solution was dropped was centrifuged to separate the
reaction product (fine particles).

[0072] FIG. 3 shows the transmission electron micrograph and the particle
size distribution of the fine particles of Example 1. As shown in FIG. 3,
the fine particles having a uniform particle size were obtained. The
average particle size of the fine particles of Example 1 was 12.5
nm±2.6 nm. The average particle size was calculated by actually
measuring the particle sizes of (at least 300) particles in the
transmission electron microscope photograph (TEM photograph) and
averaging them. FIG. 4 shows the transmission electron microscope
photograph of one of the fine particles of Example 1. Since
regularly-spaced lattice fringes are observed across the fine particle,
the fine particle in FIG. 4 is considered as a single crystal.

[0073]FIG. 5 shows the spectrum of the fine particle shown in FIG. 4,
obtained by energy-dispersive X-ray spectroscopy (EDX). The result shown
in FIG. 5 indicates that silver and rhodium are present in one particle
in a ratio of approximately 1:1, which demonstrates that the fine
particles of Example 1 are alloy fine particles in which silver and
rhodium form a solid solution at the atomic level.

[0074]FIG. 6 shows the XRD pattern (X-ray diffraction pattern) of the
fine particles of Example 1. The fitting curve shown in FIG. 6 is a curve
obtained by assuming that the alloy fine particles of Example 1 have an
fcc structure. This fitting curve coincides approximately with that of
measured values, which indicates that the alloy fine particles of Example
1 have an fcc structure. Furthermore, each peak of the fine particles of
Example 1 appears between the peak of bulk silver and the peak of bulk
rhodium. This result also indicates that the fine particles of Example 1
are alloy fine particles in which silver and rhodium form a solid
solution at the atomic level.

Example 2

[0075] In Example 2, alloy fine particles containing silver and rhodium in
an atomic ratio of approximately 50:50 were produced by spraying the
solution 11.

[0077] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 160° C., the
metal ion solution was sprayed with a spray gun onto the reducing agent
solution. At this time, the reducing agent solution was sprayed in such a
way that the temperature of the reducing agent solution was maintained at
160° C. or higher. Next, the reducing agent solution into which
the metal ion solution was added was centrifuged to separate the reaction
product (fine particles).

Example 3

[0078] In Example 3, alloy fine particles containing silver and rhodium in
an atomic ratio of approximately 75:25 were produced by spraying the
solution 11.

[0080] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 160° C., the
metal ion solution was sprayed with a spray gun onto the reducing agent
solution. At this time, the reducing agent solution was sprayed in such a
way that the temperature of the reducing agent solution was maintained at
160° C. or higher. Next, the reducing agent solution into which
the metal ion solution was added was centrifuged to separate the reaction
product (fine particles).

Example 4

[0081] In Example 4, alloy fine particles containing silver and rhodium in
an atomic ratio of approximately 25:75 were produced by spraying the
solution 11.

[0083] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 160° C., the
metal ion solution was sprayed with a spray gun onto the reducing agent
solution. At this time, the reducing agent solution was sprayed in such a
way that the temperature of the reducing agent solution was maintained at
160° C. or higher. Next, the reducing agent solution into which
the metal ion solution was added was centrifuged to separate the reaction
product (fine particles).

[0084] FIG. 7 shows the transmission electron micrographs of the fine
particles of Examples 2 to 4. As shown in FIG. 7, the particle size
increases as the proportion of silver increases under the same
conditions.

[0085]FIG. 8 shows the XRD patterns of the fine particles of Examples 2
to 4. FIG. 8 also shows the results of silver fine particles and rhodium
fine particles. The results shown in FIG. 8 indicate that the fine
particles of Examples 2 to 4 are solid solution alloy fine particles and
that all of the fine particles of Examples 2 to 4 have an fcc structure.
FIG. 9 shows the lattice constants estimated from the results of the
X-ray diffraction measurements. As shown in FIG. 9, the lattice constant
increases continuously as the silver content increases.

[0086]FIG. 10 shows the measurement results of the absorption spectra of
the fine particles of Examples 2 to 4. FIG. 10 also shows the absorption
spectra of silver fine particles and rhodium fine particles. In the
absorption spectrum of silver fine particles, an absorption peak due to
the surface plasma absorption appears around 400 nm. On the other hand,
in the cases of the fine particles of Examples 2 to 4, the absorption
peak shifts to the shorter wavelengths and becomes broader as the rhodium
content increases. This result also suggests that silver-rhodium alloy
fine particles in which silver and rhodium form a solid solution at the
atomic level were obtained.

[0087] The above results demonstrate that the fine particles of Examples 1
to 4 are solid solution alloy fine particles in which silver and rhodium
are mixed at the atomic level.

Comparative Example 1

[0088] In Comparative Example 1, fine particles were produced by adding
the solution 11 to the liquid 12 in advance and then the resulting mixed
solution was heated from about room temperature (about 20° C.) to
140° C.

[0090] Next, the metal ion solution was added to the reducing agent
solution, and then the resulting mixed solution was heated to 140°
C. with stirring. Then, the mixed solution was stirred for one hour with
its temperature maintained at 140° C. Next, after the reaction,
the mixed solution was centrifuged to separate the reaction product (fine
particles).

[0091] FIG. 11 shows the XRD pattern of the fine particles of Comparative
Example 1. FIG. 11 also shows a curve fitted using fitting components 1
and 2. The fitting component 1 is a component having a lattice constant
of 4.08 angstroms and a particle size of 9.7 nm. The fitting component 2
is a component having a lattice constant of 3.73 angstroms and a particle
size of 1.1 nm. The lattice constant of the fitting component 1 is close
to that of bulk silver (i.e., 4.086 angstroms) and the lattice constant
of the fitting component 2 is close to that of bulk rhodium (i.e., 3.803
angstroms). From the result shown in FIG. 11, it is believed that the
fine particles of Comparative Example 1 are core-shell fine particles
having a silver core or fine particles in which silver and rhodium are
phase-separated.

[0093] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 140° C., the
metal ion solution was dropped with a syringe into the reducing agent
solution. At this time, the reducing agent solution was dropped in such a
way that the temperature of the reducing agent solution was maintained at
140° C. Next, the reducing agent solution into which the metal ion
solution was dropped was centrifuged to separate the reaction product
(fine particles).

[0094] FIG. 12 shows the XRD pattern of the fine particles of Comparative
Example 2. FIG. 12 also shows a curve fitted using fitting components 1
and 2. The fitting component 1 is a component having a lattice constant
of 4.04 angstroms and a particle size of 5.3 nm. The fitting component 2
is a component having a lattice constant of 3.89 angstroms and a particle
size of 3.5 nm. In the XRD pattern of the fine particles of Comparative
Example 2, the peak is not that of a single fcc structure but is composed
of two components. The lattice constant of the fitting component 1 is
close to that of silver, and the lattice constant of the fitting
component 2 is close to that of bulk rhodium. Therefore, it is believed
that silver and rhodium are phase-separated in the fine particles of
Comparative Example 2.

[0096] Next, the metal ion solution at room temperature was dropped with a
syringe into the reducing agent solution at room temperature (about
20° C.). Next, the reducing agent solution into which the metal
ion solution was dropped was heated to reflux with stirring at
160° C. for one hour. Next, the heated solution was centrifuged to
separate the reaction product (fine particles). FIG. 13 shows the X-ray
diffraction pattern of the fine particles thus obtained. In FIG. 13, the
fitting component 1 is a component having a lattice constant of 4.070
angstroms and a particle size of 5.4 nm. The fitting component 2 is a
component having a lattice constant of 3.842 angstroms and a particle
size of 1.6 nm. From the X-ray diffraction pattern of FIG. 13, it is
believed that silver and rhodium are phase-separated in the fine
particles of Comparative Example 3.

Example 5

[0097] In Example 5, gold-rhodium alloy fine particles were produced by
dropping the solution 11.

[0099] Next, the reducing agent solution was heated, and when the
temperature of the reducing agent solution reached 160° C., the
metal ion solution was sprayed with a spray gun onto the reducing agent
solution. At this time, the reducing agent solution was sprayed in such a
way that the temperature of the reducing agent solution was maintained at
160° C. or higher. Next, the reducing agent solution onto which
the metal ion solution was sprayed was centrifuged to separate the
reaction product (fine particles of Example 5).

[0100] FIG. 14 shows the X-ray diffraction pattern of the fine particles
of Example 5. FIG. 15 shows the EDX spectrum of the fine particles of
Example 5. FIG. 15 also shows an electron micrograph of a measured fine
particle. Not only the XRD pattern of Example 5 indicates a single fcc
pattern but also its lattice constant has a value between the lattice
constant of gold nanoparticles and that of rhodium nanoparticles. These
facts prove that gold and rhodium form a solid solution at the atomic
level. Furthermore, the EDX spectrum confirms that both of the elements,
i.e., gold and rhodium, are present in one particle.

[0101] [Observation with STEM]

[0102] The silver-rhodium alloy fine particles of Example 1 were observed
using a scanning transmission electron microscope (STEM). FIG. 16A and
FIG. 16B show the data of the fine particles of Example 1. In FIG. 16A,
a) shows a dark-field STEM image, and b) to d) show elemental mapping
data. FIG. 16B shows the result of line analysis. A scale bar in each of
the images in FIG. 16A indicates 10 nm. FIG. 16A shows that all of the
particles form a solid solution. Furthermore, FIG. 16B shows that the
individual elements are not locally present in a particle but both of the
elements are uniformly distributed across the particle. In other words,
the data of FIG. 16A and FIG. 16B indicate that silver and rhodium form a
solid solution at the atomic level in the fine particles of Example 1.

[0103] The fine particles of Example 5 were observed using a STEM. FIG.
17A and FIG. 17B show the data thus obtained. In FIG. 17A, a) shows a
dark-field STEM image, and b) to d) show elemental mapping data. FIG. 17B
shows the result of line analysis. A scale bar in each of the images in
FIG. 17A indicates 10 nm. FIG. 17A shows that all of the particles form a
solid solution. Furthermore, FIG. 17B shows that the individual elements
are not locally present in a particle but both of the elements are
uniformly distributed across the particle. In other words, the data of
FIG. 17A and FIG. 17B indicate that gold and rhodium form a solid
solution at the atomic level in the fine particles of Example 5.

[0104] As shown in the above examples, according to the production method
of the present invention, silver-rhodium fine particles in which silver
and rhodium form a solid solution and gold-rhodium fine particles in
which gold and rhodium form a solid solution were obtained. No data have
been presented to indicate that these elements are mixed at the atomic
level. The present inventors have presented the first data indicating
that these elements are mixed at the atomic level.

INDUSTRIAL APPLICABILITY

[0105] According to the present invention, solid solution alloy fine
particles in which a plurality of metal elements are mixed at the atomic
level are obtained. These alloy fine particles can be used for various
applications (for example, catalysts). For example, silver-rhodium alloy
fine particles can be used as a catalyst for organic synthesis, an
electrode catalyst for a fuel cell, and a catalyst for reducing NOx.
Furthermore, since silver-rhodium alloy fine particles are considered to
exhibit hydrogen storage properties, they are expected to be applied to
various devices by taking advantage of their hydrogen storage properties.
Silver-rhodium alloy fine particles in which silver and rhodium form a
solid solution at the atomic level are expected to exhibit the properties
similar to those of palladium. Likewise, it is possible to produce alloys
having various properties by producing alloys of various elements.

Patent applications by JAPAN SCIENCE AND TECHNOLOGY AGENCY

Patent applications in class ALL METAL OR WITH ADJACENT METALS

Patent applications in all subclasses ALL METAL OR WITH ADJACENT METALS